1. Field of the Invention
The present invention relates to a photoelectric conversion device suitable for a photometry sensor of a camera, an image sensor of an image reading apparatus such as a facsimile or copying machine, or a light-receiving sensor of an optical communication apparatus or the like and, more particularly, to a method of manufacturing a photoelectric conversion device according to an isolation technique which does not cause degradation in characteristics of a photoelectric conversion device using an avalanche effect and a photoelectric conversion device in which elements are arranged one-dimensionally or two-dimensionally.
2. Related Background Art
Recently, elongated line sensors in which photosensors are arranged one-dimensionally and area sensors in which photosensors are arranged two-dimensionally are used frequently with the spread of image information processing apparatuses such as facsimile apparatuses, digital copying machines, image readers, and video cameras. Although various types of photosensors are usable in these line and area sensors, thin-film photosensors are applied extensively in consideration of the following functionalities:
(1) Layers can be stacked so as to isolate individual functions in accordance with the properties of a material.
(2) A large area can be formed at one time.
(3) A low-temperature process which is not limited by the type of substrate can be adopted, so that any one of a glass, a semiconductor substrate, and a metal is usable as a substrate.
(4) A formation process is simple, and this realizes a low cost.
As the structure of an element, on the other hand, photodiode structures which suppress a dark current by preventing injection of carriers from electrodes are used widely because a high sensitivity can be obtained by a high S/N ratio.
Among other photodiode type photosensors, a so-called PIN photodiode is the mainstream in favor of suppression of a dark current, in which a photoconductive film is formed by arranging heavily doped impurity layers (P and N layers) having opposite conductivity types on the upper and lower surfaces of a semiconductor layer (an I layer) not doped with or doped slightly with an impurity for controlling a conductivity type.
To realize a one-dimensional or two-dimensional arrangement of the photodiode type photosensors, such as the arrangement of a line sensor or an area sensor, at least one of upper and lower electrodes is isolated into individual pixels to form discrete electrodes in order for a plurality of photosensors to be able to operate independently. Especially in the PIN photodiode, in order to suppress crosstalk, a heavily doped impurity layer of a photoconductive film formed on the side of discrete electrodes must also be isolated into individual pixels.
FIGS. 1 and 2 are schematic sectional views showing examples of conventional area sensors. FIG. 1 illustrates a sensor in which discrete electrodes are connected to a lower electrode, and FIG. 2 illustrates a sensor in which discrete electrodes are connected to an upper electrode.
If discrete electrodes 115 are arranged on the side of an upper electrode 113 of a photoconductive film in an area sensor as shown in FIG. 2, the ratio of an effective pixel region B to a region A of one pixel decreases, making it impossible to take advantage of the merit of a multilayered photoelectric conversion element. Therefore, as shown in FIG. 1, the discrete electrodes 115 are arranged on the side of a lower electrode 107.
FIG. 3 is a schematic sectional view showing another example of a conventional line sensor. Also in a line sensor, as shown in FIG. 3, to prevent occurrence of a pixel defect caused by disconnection of a wiring electrode 115 connected to a signal processing element at a position indicated by X, it is advantageous to use the discrete electrodes 115 as a lower electrode. Therefore, pixel isolation methods (to be referred to as isolation methods hereinafter) used when discrete electrodes are formed on the lower electrode side are important.
The isolation methods are roughly classified into two categories as described below.
The first one is a method of isolating only a heavily doped impurity layer (a lower heavily doped impurity layer) on the side of discrete electrodes into individual pixels. In this method, a film for discrete electrodes and a lower heavily doped impurity layer are deposited and then isolated. Thereafter, an I layer, an upper heavily doped impurity layer, and an upper electrode layer are deposited to form photodiodes.
The second one is a method by which not only a lower heavily doped impurity layer but also a whole photoconductive film including an I layer and an upper heavily doped impurity layer is isolated. That is, a film for discrete electrodes is deposited and patterned into a desired shape, and a lower heavily doped impurity layer, an I layer, and an upper heavily doped impurity layer, which constitute a photoconductive film, are deposited in succession. Thereafter, all the layers of this photoconductive film are isolated successively.
In video information systems, optical communications, various other industries, and fields of industrial apparatuses, which use light as a medium of information signals, a semiconductor light-receiving element for converting a light signal into an electrical signal is one constituent element that is most important and basic, and a large number of semiconductor light-receiving elements have been put into practical use. The semiconductor light-receiving element is generally required to have a high signal-to-noise ratio in its photoelectric conversion characteristics.
An avalanche photodiode (to be abbreviated as an APD hereinafter) which uses an avalanche effect is a promising candidate for a semiconductor light-receiving element meeting the above requirement because the avalanche diode has a high gain and a high response speed.
A large number of APDs using compound semiconductors, such as InGaAs, as their materials have already been put to use as semiconductor light-receiving elements particularly in optical communication systems. In addition, development for improving the basic characteristics of the elements, such as low noise, a high response speed, and a high gain, has been advanced, so applications of the APDs to other fields, e.g., a visible light-receiving element, are also expected.
FIG. 4 is a longitudinal sectional view showing the structure of a conventional APD for optical communications.
Referring to FIG. 4, this APD comprises an n.sup.+ -type InP layer 151, an n-type InGaAs layer 152, an n-type InP layer 153, and a p.sup.+ -type InP layer 154. The n-type InGaAs layer 152, the n-type InP layer 153, and the p.sup.+ -type InP layer 154 from a mesa shape. A p-type electrode 156 is formed on the upper surface of the p.sup.+ -type InP layer 154 except for a window 155. An n-type electrode 157 is formed on the lower surface of the n.sup.+ -type InP layer 151. This APD also includes a passivation film 158. When the p-type electrode 156 and the n-type electrode 157 are reverse-biased and light is radiated through the window 155, this light is absorbed by the n-type InGaAs layer 152 (which serves as a light absorption layer), causing photoelectric conversion. That is, electrons and holes formed by the n-type InGaAs layer 152 to constitute pairs transit toward the n-type electrode 157 and the p-type electrode 156, respectively. Since the n-type InP layer 153 (which serves as a multiplication layer) has a strong electric field, an avalanche phenomenon in which a large number of electron-hole pairs are formed takes place while the holes are running, giving rise to a multiplication effect for forming a plurality of electron-hole pairs for one photon. This consequently makes it possible to detect even weak incident light. In this conventional structure, however, a practical multiplication factor is small, approximately two times. In addition, excess multiplication noise is generated by fluctuations inherent in the multiplication process, leading to a decrease in the S/N ratio.
As an APD for optical communications made in consideration of these two problems, F. Capasso et al. have proposed a low-noise APD which is manufactured by using primarily a Group III-V compound semiconductor in accordance with, e.g., a molecular beam epitaxy (MBE) method, and which can be used in optical communication systems, in Japanese Laid-Open Patent Application No. 58-157179 or IEEE Electron Device Letters, the EDL 3rd ed. (1982), pp. 71-73.
The characteristic feature of this element is a multilayered heterojunction structure which is formed by stacking semiconductor layers whose band gap is changed continuously from a narrow side to a broad side by changing the composition ratio of their material (for example, if the material is a Group III-V compound semiconductor, the composition ratio of a Group III semiconductor to a Group V semiconductor), and which encourages ionization by using a step transition portion (to be abbreviated as a step-back structure hereinafter) of an energy band formed. A schematic structure of the element proposed will be described below with reference to FIGS. 5A to 5C.
FIG. 5A is a longitudinal sectional view of this element. Referring to FIG. 5A, five step-back structure layers 201, 203, 205, 207, and 209 serving as a multiplication layer are sandwiched between a p-type semiconductor layer 211, which serves as a light absorption layer, and an n-type semiconductor layer 215. An electrode 213 is in ohmic contact with the p-type semiconductor layer 211, and an electrode 214 is in ohmic contact with the n-type semiconductor layer 215.
FIG. 5B is a view showing the energy band structure of band-gap inclined layers of this element when no bias is applied. In FIG. 5B, three band-gap inclined layers are illustrated. Each layer has a composition by which a band gap is changed linearly from a narrow band gap Eg.sub.2 to a wide band gap Eg.sub.3.
The sizes of step backs of a conduction band and a valence band are indicated by .DELTA.Ec and .DELTA.Ev, respectively. As will be described later, the .DELTA.Ec is set to be larger than the .DELTA.Ev principally in order to facilitate ionization of electrons.
FIG. 5C is a view showing the energy band structure of this element when a reverse bias voltage is applied to the element. Note that this reverse bias voltage need not be a strong electric field compared to the APD described above with reference to FIG. 4.
When light is incident from the p-type semiconductor layer 211, this light is absorbed by the p-type semiconductor layer and the individual step-back structure layers, and photoelectric conversion is performed in the same manner as in the above-mentioned APD. Electrons and holes thus formed to constitute pairs transit toward the n-type semiconductor layer 215 and the p-type semiconductor layer 211, respectively. The difference of this element from the APD shown in FIG. 4 is that free electrons are produced when the energy step .DELTA.Ec (in the case of electrons; the .DELTA.Ev in the case of holes) of each step-back structure increases to be higher than the ionization energy, and this produces electron-hole pairs, bringing about a multiplying effect. Since the individual step-back structure layers have the identical function, a multiplication of 2.sup.n, of course, results for a layer number n. As an example, by setting ideally .DELTA.Ec&gt;.DELTA.Ev.congruent.0, it is possible to reduce the ionization degree of holes to be much smaller than that of electrons.
That is, since energy discontinuity in heterojunction portions 202, 204, and 206 in which the band gap steps back abruptly helps ionization, ionization occurs selectively in the vicinity of this step back to multiply carriers.
With this structure, fluctuations in a portion where ionization takes place are reduced, and this reduces fluctuations inherent in the multiplication process. This consequently makes it possible to realize a low-noise APD which is improved in an S/N ratio by reducing excess noise, and which therefore can be used in optical communication systems.
In the manufacture of the PIN type photodiode as described above, however, an isolation step is introduced to drive a plurality of elements simultaneously. This conventionally gives rise to a problem that device characteristics, particularly a dark current characteristic, is degraded significantly as compared with that obtained by a single element. FIG. 6 shows a comparison between the dark current-voltage characteristics of an element (a) formed through the isolation step and an element (b) not subjected to the isolation step. As shown in FIG. 6, the ratio of an increase in the dark current to an increase in the voltage is higher in the element (a) subjected to the isolation step; that is, the increase is 10 to 100 times larger, as a current value, for an applied voltage of 5 V.
This phenomenon will be described below by taking the above two types of the isolation methods as examples.
In the first method, i.e., the method of isolating only a lower heavily doped impurity layer, after a lower heavily doped impurity layer is deposited on a substrate in a vacuum chamber, the substrate is removed from the vacuum chamber before being subjected to photolithography. Therefore, the surface of the lower heavily doped impurity layer is exposed to the atmosphere and subjected to coating and peeling of a photoresist. The result is a large number of structural defects of a semiconductor formed in the interface between the lower heavily doped impurity layer and an I layer.
Such structural defects will be described with reference to FIGS. 7A to 7C. FIGS. 7A, 7B, and 7C are a plan view, a sectional view, and an energy band diagram, respectively, of a PIN photodiode. Since these structural defects function as formation centers of carriers as indicated by X in FIGS. 7B and 7C, an applied voltage to the PIN photodiode increases. Therefore, an abrupt increase in a dark current is brought about if an electric field applied to the interface is enhanced.
In the second method, i.e., the method of isolating a whole photoconductive film, there is no increase in a dark current caused by formation of carriers in the interface. However, as shown in FIGS. 8A to 8C, this isolation produces edges in portions surrounding pixels as indicated by X even in an I layer.
FIGS. 8A, 8B, and 8C are a plane view, a sectional view, and an energy band diagram, respectively, of this PIN photodiode.
Structural defects of a semiconductor also exist in such an edge portion, but the quantity of defects changes greatly in accordance with a formation process of edges. For example, if edges are formed by performing isolation through reactive ion etching, a large number of structural defects are produced in the same manner as described above by physical shocks of ions.
If, on the other hand, edges are formed by performing isolation through wet-etching, a defect level increases due to formation of a discontinuous portion in a semiconductor lattice. This increase, however, is very small compared to that caused by the physical shocks during the reactive ion etching, and so an increase in the dark current is only about several times that when no isolation is performed.
It is, however, often common practice to use a strong acid in the wet-etching for photoconductive films, and a photoresist regularly used has only a resistance to such an extent that it can withstand the etching for about one or two minutes, while a time required to complete the isolation is 10 to 20 minutes. That is, the wet-etching is practically inapplicable as the method of isolation.
For this reason, the formation of edges is generally performed by the reactive ion etching, and so a number of structural defect levels result. Since all of these structural defects serve as production centers of carriers and an electric field is applied to an I layer at any instant, a resulting dark current is very large.
A lift-off process is also usable as the method of isolating a whole photoconductive film. In this case, however, in order to remove unnecessary portions from a photoconductive film, deposition of the photoconductive film is performed with an organic resist coated on a substrate. Therefore, large quantities of impurities such as O, C, and N are mixed in the photoconductive film, producing a number of impurity levels, which cause a dark current, throughout the film.
As described above, when the conventional PIN photodiodes are arranged one-dimensionally or two-dimensionally to be used as a line sensor or an area sensor, it is difficult to prevent a degradation in characteristics, i.e., an increase in a dark current.
In addition, there are several technical problems to be solved to put the APD explained with reference to FIGS. 5A to 5D into practical use.
Technical problems to be solved in respect of performance of the element are as follows.
(1) Since incident light is absorbed by the p-type semiconductor layer and the multiplication layer, the multiplication factor changes in accordance with the wavelength of the incident light. Therefore, this APD is unsuitable as a reading element.
(2) Since the forbidden band widths of the light absorption layer and the multiplication layer are small, a dark current during operation is large, and this leads to large noise.
(3) The material of the APD is limited because the APD is used in optical communications. Therefore, the wavelength of light which the APD can respond to is approximately 800 to 1,600 nm, i.e., the APD cannot respond to light having another wavelength, e.g., visible light.
Technical problems posed in the manufacture of the element are as follows.
(1) In the formation of the step-back structure using a compound semiconductor, composition modulation is difficult to perform, and the sizes of the .DELTA.Ec and .DELTA.Ev are limited. This imposes limitations on noise reduction.
(2) Group III-V and II-IV compound semiconductors used as materials have problems as industrial materials, such as toxicity and cost.
(3) A compound semiconductor formation method requires an ultra high vacuum and film formation at high temperatures (approximately 500 to 650.degree. C.) and also has difficulty in formation of films having large areas. These problems make this method unsuitable as a method of manufacturing a reading element.
Furthermore, when the APD is used as a solid-state image pickup element, noise is produced by leakage of a dark current between pixels if a plurality of APDs are unisolated.
This element isolation is conventionally performed by etching all layers of the APD by using, e.g., reactive ion etching. However, this reactive ion etching poses various problems, such as degradation of device performance, so some improvement has been requested.
The above APD is useful as a discrete light-receiving element for optical communications which operates upon application of a strong electric field. If, however, this APD is to be used extensively as a photoelectric conversion device, such as a video camera or a scanner, which performs a storage operation, the following problems arise in some cases.
(1) Since the conventional APD uses a Group III-V or II-VI compound semiconductor as its constituent material, problems such as toxicity and cost of such a material as an industrial material are brought about.
(2) The formation of a single-crystal compound semiconductor as the material of the APD requires film formation using an ultra-high-vacuum apparatus at high temperatures (about 500.degree. C. or more). It is therefore difficult to apply the element to a photoelectric conversion device with a large area and is impossible to stack the element on a semiconductor substrate on which a signal processing circuit or the like is already formed. This limits the range of applications of the APD.
(3) To realize a low-noise APD, it is necessary to increase the ionization degree of a step-back heterojunction portion, and, for this purpose, use of a material in which energy discontinuity of a step-back portion in only one of a valence band and a conduction band is large is required. However, crystalline compound semiconductors meeting this requirement are limited. Furthermore, to realize an APD of lower noise in which a thermally generated dark current which causes noise is also reduced, the above requirement must be satisfied by the use of a material having a large minimum forbidden band width (desirably higher than 1.0 eV). No crystalline compound semiconductor meeting this requirement exists.
(4) When a storage operation is performed, an electric field applied to the APD decreases with an increase in a carrier storage amount. As shown in FIG. 49C, therefore, a spike and a notch are produced in the step-back heterojunction portion of the multiplication layer consisting of an I-type semiconductor. This decreases effective band discontinuity in the step-back heterojunction portion to decrease the ionization degree and also produces energy discontinuity in a direction in which transition of carriers is inhibited. The results are a decrease in the multiplication factor, deterioration in the linearity of an incident light quantity-to-output characteristic, and a reduction in a response speed.
(5) When a plurality of carrier multiplication layers are formed by using a non-single-crystal material such that a forbidden band is inclined continuously, diffusion of hydrogen in a film occurs due to the difference in bonding energy or the like in a heterojunction portion between a minimum forbidden band and a maximum forbidden band. As a result, defect levels such as dangling bonds are produced to cause recombination of carriers, and this may lead to a decrease in the multiplication factor.